Use `std::optional` in `CValidationInterface::GetAddressForMining`

The initial value for the provided return value is now `std::nullopt`
instead of the default value of the `MinerAddress` variant. This makes
it easier to distinguish "no miner address was provided" from "invalid
miner address".

src/miner.cpp

@@ -726,7 +726,7 @@ std::optional<MinerAddress> ExtractMinerAddress::operator()(const libzcash::Unif
 }
 
 
-void GetMinerAddress(MinerAddress &minerAddress)
+void GetMinerAddress(std::optional<MinerAddress> &minerAddress)
 {
     KeyIO keyIO(Params());
 
@@ -804,8 +804,8 @@ void static BitcoinMiner(const CChainParams& chainparams)
     // Each thread has its own counter
     unsigned int nExtraNonce = 0;
 
-    MinerAddress minerAddress;
-    GetMainSignals().AddressForMining(minerAddress);
+    std::optional<MinerAddress> maybeMinerAddress;
+    GetMainSignals().AddressForMining(maybeMinerAddress);
 
     unsigned int n = chainparams.GetConsensus().nEquihashN;
     unsigned int k = chainparams.GetConsensus().nEquihashK;
@@ -826,9 +826,10 @@ void static BitcoinMiner(const CChainParams& chainparams)
 
     try {
         // Throw an error if no address valid for mining was provided.
-        if (!std::visit(IsValidMinerAddress(), minerAddress)) {
+        if (!(maybeMinerAddress.has_value() && std::visit(IsValidMinerAddress(), maybeMinerAddress.value()))) {
             throw std::runtime_error("No miner address available (mining requires a wallet or -mineraddress)");
         }
+        auto minerAddress = maybeMinerAddress.value();
 
         while (true) {
             if (chainparams.MiningRequiresPeers()) {

src/miner.h

@@ -89,7 +89,7 @@ CBlockTemplate* CreateNewBlock(const CChainParams& chainparams, const MinerAddre
 
 #ifdef ENABLE_MINING
 /** Get -mineraddress */
-void GetMinerAddress(MinerAddress &minerAddress);
+void GetMinerAddress(std::optional<MinerAddress> &minerAddress);
 /** Modify the extranonce in a block */
 void IncrementExtraNonce(
     CBlockTemplate* pblocktemplate,

src/rpc/mining.cpp

@@ -186,8 +186,14 @@ UniValue generate(const UniValue& params, bool fHelp)
     int nHeight = 0;
     int nGenerate = params[0].get_int();
 
-    MinerAddress minerAddress;
-    GetMainSignals().AddressForMining(minerAddress);
+    std::optional<MinerAddress> maybeMinerAddress;
+    GetMainSignals().AddressForMining(maybeMinerAddress);
+
+    // Throw an error if no address valid for mining was provided.
+    if (!(maybeMinerAddress.has_value() && std::visit(IsValidMinerAddress(), maybeMinerAddress.value()))) {
+        throw JSONRPCError(RPC_INTERNAL_ERROR, "No miner address available (mining requires a wallet or -mineraddress)");
+    }
+    auto minerAddress = maybeMinerAddress.value();
 
     // If the keypool is exhausted, no script is returned at all.  Catch this.
     auto resv = std::get_if<boost::shared_ptr<CReserveScript>>(&minerAddress);
@@ -195,11 +201,6 @@ UniValue generate(const UniValue& params, bool fHelp)
         throw JSONRPCError(RPC_WALLET_KEYPOOL_RAN_OUT, "Error: Keypool ran out, please call keypoolrefill first");
     }
 
-    // Throw an error if no address valid for mining was provided.
-    if (!std::visit(IsValidMinerAddress(), minerAddress)) {
-        throw JSONRPCError(RPC_INTERNAL_ERROR, "No miner address available (mining requires a wallet or -mineraddress)");
-    }
-
     {   // Don't keep cs_main locked
         LOCK(cs_main);
         nHeightStart = chainActive.Height();
@@ -569,9 +570,14 @@ UniValue getblocktemplate(const UniValue& params, bool fHelp)
     if (IsInitialBlockDownload(Params().GetConsensus()))
         throw JSONRPCError(RPC_CLIENT_IN_INITIAL_DOWNLOAD, "Zcash is downloading blocks...");
 
+    std::optional<MinerAddress> maybeMinerAddress;
+    GetMainSignals().AddressForMining(maybeMinerAddress);
 
-    MinerAddress minerAddress;
-    GetMainSignals().AddressForMining(minerAddress);
+    // Throw an error if no address valid for mining was provided.
+    if (!(maybeMinerAddress.has_value() && std::visit(IsValidMinerAddress(), maybeMinerAddress.value()))) {
+        throw JSONRPCError(RPC_INTERNAL_ERROR, "No miner address available (getblocktemplate requires a wallet or -mineraddress)");
+    }
+    auto minerAddress = maybeMinerAddress.value();
 
     static unsigned int nTransactionsUpdatedLast;
     static std::optional<CMutableTransaction> cached_next_cb_mtx;

src/validationinterface.h

@@ -42,7 +42,7 @@ protected:
     virtual void Inventory(const uint256 &hash) {}
     virtual void ResendWalletTransactions(int64_t nBestBlockTime) {}
     virtual void BlockChecked(const CBlock&, const CValidationState&) {}
-    virtual void GetAddressForMining(MinerAddress&) {};
+    virtual void GetAddressForMining(std::optional<MinerAddress>&) {};
     virtual void ResetRequestCount(const uint256 &hash) {};
     friend void ::RegisterValidationInterface(CValidationInterface*);
     friend void ::UnregisterValidationInterface(CValidationInterface*);
@@ -67,7 +67,7 @@ struct CMainSignals {
     /** Notifies listeners of a block validation result */
     boost::signals2::signal<void (const CBlock&, const CValidationState&)> BlockChecked;
     /** Notifies listeners that an address for mining is required (coinbase) */
-    boost::signals2::signal<void (MinerAddress&)> AddressForMining;
+    boost::signals2::signal<void (std::optional<MinerAddress>&)> AddressForMining;
     /** Notifies listeners that a block has been successfully mined */
     boost::signals2::signal<void (const uint256 &)> BlockFound;
 };

src/wallet/wallet.cpp

@@ -5547,7 +5547,7 @@ void CWallet::UpdatedTransaction(const uint256 &hashTx)
     }
 }
 
-void CWallet::GetAddressForMining(MinerAddress &minerAddress)
+void CWallet::GetAddressForMining(std::optional<MinerAddress> &minerAddress)
 {
     if (!GetArg("-mineraddress", "").empty()) {
         return;

src/wallet/wallet.h

@@ -1719,7 +1719,7 @@ public:
         }
     }
 
-    void GetAddressForMining(MinerAddress &minerAddress);
+    void GetAddressForMining(std::optional<MinerAddress> &minerAddress);
     void ResetRequestCount(const uint256 &hash)
     {
         LOCK(cs_wallet);